An infrared photodetector for use as a focal plane array can comprise a “barrier” layer, whose composition is specifically chosen to produce a near-zero band offset for the minority carries, but serves to block the majority carriers. Prior-art devices rely upon undoped or uniformly doped bandgap AlSb-based alloys in the barrier layer, which are difficult to controllably dope, leading to detector turn-on voltage issues, and spillage of the electric field into the narrow-bandgap absorber region of the detector structure resulting in increased generation-recombination (G-R) dark current
Devices can be designed to achieve lower dark current and desired operating characteristics through judicious device design. In addition, low voltage operation of barrier-based detectors is desired for two reasons: 1) compatibility with the Read Out Integrated Circuit (ROIC) drive voltage needed to turn-on the diode, which is typically less than 500 mV, and 2) to ensure that barriers (for the majority carriers) that have been introduced to minimize dark current do not block the transport of minority carriers; inadvertently introduced minority carrier barriers can adversely affect the quantum efficiency at a given bias. An operating bias (the bias at which the photo-response reaches the near maximum value) of zero volts indicates that the device is free of such minority carrier barrier. This represents the ideal alignment of the energy bands between the absorber, barrier and the contact layer. Hence zero-bias operation verifies absence of undesirable minority carrier barriers. Additionally it is desired to achieve the lowest dark current at the operating bias while suppressing any quantum efficiency losses.
An example prior art photodetector “High Operating Temperature XBn-InAsSb Bariode Detectors”, Philip Klipstein, Olga Klin, Steve Grossman, Noam Snapi, Inna Lukomsky, Michael Yassen, Daniel Aronov, Eyal Berkowitz, Alex Glozman, Osnat Magen, Itay Shtrichman, Rami Frenkel and Eliezer Weiss, Quantum Sensing and Nanophotonic Devices IX, edited by Manijeh Razeghi, Eric Tournie, Gail J. Brown, Proc. of SPIE Vol. 8268, 82680U•© 2012 SPIE recommends doping the absorber layer the same as the barrier layer, contrary to the principles of the present invention.
U.S. Pat. No. 8,004,012 to Klipstein, U.S. Pat. Nos. 7,795,640, and 7,687,871 to Maimon teach an absorber layer doped the same type as the barrier layer.
One consequence of doping the absorber layer the same as barrier layer for the prior art devices is that they exhibit variability in the bias needed to operate, and potential increase in the detector dark current due to the electric field spilling into the absorber.
FIG. 1A shows the band diagram for the structure diagram in FIG. 1B. The background doping from the Molecular Beam Epitaxy (MBE) or metallorganic chemical vapor deposition (MOCVD) system results in the formation of a hole-barrier (102) in a device with a not-intentionally doped barrier (101); the barrier has an n-type background conductivity. The barrier to hole transport (102) reduces the quantum efficiency (QE) of the detector.
FIG. 2 shows the energy band diagram of a prior art photodetector and the effect of various bias voltages and background doping. The background doping of the epitaxial growth system (MBE or MOCVD) can result in the formation of a hole-barrier 201 in the valence band (and 102 region in FIG. 1A) in a device with a not-intentionally doped barrier. This results in the following disadvantage: 1) large turn on voltages >400 mV to undo the barrier (207 curve) which approaches the limits of a ROIC's capability, and can diminish the quantum efficiency (QE) of the device, and 2) spilling of the electric field (due to the large applied bias) into the absorber which increases the G-R dark current (see FIG. 4 for details).
FIG. 3A shows the energy band diagram and FIG. 3B shows the corresponding structure of a prior art photodetectors with counter doping of the barrier (i.e. doped p-type). Counter doping of the barrier p-type reduces the hole barrier to about 100 mV, as a result of the band misalignment between the absorber and the n+ contact layer. However, the bias needed to surmount the 100 mV barrier results in the field spilling into the absorber as shown in FIG. 4.
FIG. 4 shows the energy band diagram of a prior art infrared photodetector of FIG. 3B with a doped p-type barrier and illustrating the spill over of the electric field in the absorber region when biased to overcome the hole barrier in the conduction band. The bias needed to undo the small (˜100 mV) barrier results in the spilling of the electric field into the absorber (region 401) which increases the G-R dark current.
The problems described with respect to the prior art are at least partially solved by the embodiments according to the principles of the present invention herein.